How to map properties with a type System.Type?

cloudster · **Joined:** Sat Jul 07, 2007 8:46 pm **Posts:** 6

I have a class:

Code:

 public class Comment
    {
        #region private fields
        private int? id;
        private string text;
        private DateTime createdOn;
        private User createdBy;
        private object objectId;
        private Type objectType;
        #endregion

        #region public properties
        public Comment()
        {
            CreatedOn = DateTime.Now;
        }

        virtual public int? Id
        {
            get { return id; }
            set { id = value; }
        }

        virtual public string Text
        {
            get { return text; }
            set { text = value; }
        }

        virtual public DateTime CreatedOn
        {
            get { return createdOn; }
            set { createdOn = value; }
        }

        virtual public User CreatedBy
        {
            get { return createdBy; }
            set { createdBy = value; }
        }

        virtual public object ObjectId
        {
            get { return objectId; }
            set { objectId = value; }
        }

        virtual public Type ObjectType
        {
            get { return objectType; }
            set { objectType = value; }
        }
        
        #endregion
    }

Bascally a POCO object with a property typeof System.Type.

In my mapping file:

Code:

But I always get exception "Object reference not set to an instance of an object." when I tries to load a collection.

Have I done something wrong?

Thanks.

marcal · **Posted:** Wed Jul 11, 2007 11:10 am

Need more info. Doesn't look like this property is the problem, though.

IgorK · **Posted:** Wed Jul 11, 2007 3:03 pm

Hi cloudster!

What would you store in DB for this "type" and what is more interesting - for objectId?
Are you really want to create association from this class to EVERY other possible persistent class? But then I believe you have to create some sort of common interface, or even common (abstract) base class for every other persistent class in whole application - to map ObjectId to this class/interface... And of course you have to use one common sequence of values for ids - uuid for example.

cloudster · **Joined:** Sat Jul 07, 2007 8:46 pm **Posts:** 6

Hi Igork,

This is my way to solve the lack-of-mixin problem in C#. I am developing a web application.
I have interfaces like IFileAttachable, ICommentable, ITaggable.
All the interfaces have no member.

If the current object is ICommentable, my controller will automatically use ICommentService to load the comment from db. like this:

Code:

     public ICollection<Comment> GetComments(ICommentable obj)
        {
            Type objType = obj.GetType();

            return repository.FindAll(Where.Comment.ObjectId == obj.Id
                && Where.Comment.objectTypeString == objType.AssemblyQualifiedName
                && Where.Comment.AddOrder(OrderBy.Comment.CreatedOn.Asc)
            );
        }

Then a comment section will be rendered automatically if the current object is ICommentable.

So in db side, my comments table is not associated with any other classes. I have to load/save comments basing on the current object. Before, I have this work perfectly if I map the Type directly. I changed to nhibernate trunk version and now have to convert the Type to its AssemblyQualifiedName by myself. Something strange.

IgorK wrote:

Hi cloudster!

What would you store in DB for this "type" and what is more interesting - for objectId?
Are you really want to create association from this class to EVERY other possible persistent class? But then I believe you have to create some sort of common interface, or even common (abstract) base class for every other persistent class in whole application - to map ObjectId to this class/interface... And of course you have to use one common sequence of values for ids - uuid for example.

cloudster · **Joined:** Sat Jul 07, 2007 8:46 pm **Posts:** 6

Forget to say all classes in my model layer use int as identifier for the persistent purpose. So they do have a common interface.

IgorK wrote:

Hi cloudster!

What would you store in DB for this "type" and what is more interesting - for objectId?
Are you really want to create association from this class to EVERY other possible persistent class? But then I believe you have to create some sort of common interface, or even common (abstract) base class for every other persistent class in whole application - to map ObjectId to this class/interface... And of course you have to use one common sequence of values for ids - uuid for example.

IgorK · **Posted:** Thu Jul 12, 2007 5:25 am

Hi cloudster!

Then maybe you'd better map this field to string instead of Type?

marcal · **Posted:** Thu Jul 12, 2007 1:20 pm

Igor,

See here: System.Type automatically maps to string (nvarchar) on the DB side. It works by saving the FQN of the type, I believe. But to load it, the same assembly needs to be available.

cloudster,

what is the full stack trace of the exception? If it is intermittent, it may be because NH is having difficulty loading a particular type. Maybe the comment was saved in one program for a type from a given assembly, then you tried to reload it in another program where that same assembly wasn't available? In that case, Igor may be right: you might be better off just storing the type FQN in a string on the object yourself. Less elegant, but maybe a necessary tradeoff.

cloudster · **Joined:** Sat Jul 07, 2007 8:46 pm **Posts:** 6

I am using a string field now as a workaround.

Code:

 private string objectTypeString;
 virtual public Type ObjectType
 {
    get { return Type.GetType(objectTypeString); }
    set { objectTypeString = value.AssemblyQualifiedName; }
 }

and
<property name="objectTypeString" access="field" column="[ObjectType]"/>

It works.

But I am using ayende's NHibernate Query Generator and I have to write code like

repository.FindAll(Where.Comment.ObjectId == obj.Id
&& Where.Comment.objectTypeString == objType.AssemblyQualifiedName
&& Where.Comment.AddOrder(OrderBy.Comment.CreatedOn.Asc));

I just hope I can compare the type directly, not the AssemblyQualifiedName.

Don't know why it's not working.

marcal · **Posted:** Thu Jul 12, 2007 5:42 pm

What is the full stack trace of the exception?

cloudster · **Joined:** Sat Jul 07, 2007 8:46 pm **Posts:** 6

I found I can't use

return repository.FindAll(Expression.Eq("ObjectType", objType));

return repository.FindAll() is OK.

The Positional parameter is null which means the objType is not converted to string correctly.

marcal · **Posted:** Thu Jul 12, 2007 10:55 pm

FindAll() is not familiar to me, is it yours? What is the code underlying it? Does this really throw an exception, or does it just not return any results?

cloudster · **Joined:** Sat Jul 07, 2007 8:46 pm **Posts:** 6

FindAll() is just a wrapper of List<T>(). One exception is thrown from nhibernate.dll.

"Object reference not set to an instance of an object."

Looks I can't use Expression.Eq for system.Type.